The present invention relates to a process by which ricinic compounds such as castor oil or its major constituent ricinoleic acid can be converted into sebacic acid and octanol-2 under high temperature oxidative conditions. This reaction involves the addition of an alkali in aqueous solution in the presence of a thinning agent, which will change the fluidity of the solution allowing for improved yield and efficiency of the reaction and improved purity of the products. The thinning agent should be in amount sufficient to reduce foaming and solidification of the product, while improving the yield of the reaction.
Sebacic acid is a valuable precursor for the production of nylon 6, 10. Octanol-2 is also a commercially useful chemical. The reaction to cleave castor oil in a strongly basic solution under heat to produce sebacic acid and octanol-2 is well known in the art. Addition of alkali salts at high temperatures will cause these reactants to solidify and cause foaming due to the production of soaps. This solidified reaction mixture is hard to stir by mechanical means. Various approaches have been taken to address the problem of viscous or solid soap solutions.
U.S. Pat. No. 2,674,608 to George DuPont has tried to solve the problem of excessive viscosity thickening in the cleavage reaction of castor oil by the addition of substantial amounts of phenolic compounds such as cresol and phenol. The formed alkali metal phenoxides have low volatility at 200-300xc2x0 C. and remain in the reaction mass. After acidulation of the reaction mixture, to recover sebacic acid from its salt, it has been found phenol is also liberated. Phenol is soluble in water and will contaminate the sebacic acid. Foaming is also a problem with phenol present.
U.S. Pat. No. 5,952,517 to Reis and assigned to CasChem uses heat transfer fluids such as an aromatic oil, glycol oil, a petroleum oil, a fluorocarbon oil or silicone oil as a dilutent to allow for mechanical agitation of the reaction mixture in the castor oil cleavage reaction. These fluids are typically high boiling and very difficult to recycle. Their presence as a contaminant in the sebacic acid, which is hard to remove by distillation also makes these oils unsuitable for nylon manufacture, and other high quality uses.
Other carboxylic acids have also been suggested as thinning agents in this reaction such as oleic and linoleic acids (C18). In addition, six to ten carbon unit (C6 to C10) carboxylic acids such as carproic, caprylic and capric acid have been. examined by the inventors. None of these acids has been successfully used as a thinning agent. These normal acids melt from 5xc2x0 C. to 31xc2x0 C. and their corresponding alkali salts are solid under the reaction conditions for the cleavage of the ricinic compounds. This is contrasted with the isocarboxylic acids of this invention such as isodecanoic acid or isooctanoic acid which melt from xe2x88x9250xc2x0 C. to xe2x88x92100xc2x0 C. The corresponding alkali salts of the isocarboxylic acids melt at much lower temperatures than the salts of normal acids. Therefore they act as nonvolatile thinning agents in the reaction.
The ideal thinning agent for the production of sebacic acid should be inexpensive, non-volatile and meltable under the reaction conditions, easy to recover, resistant to decomposition under the reaction conditions, and a non-foaming agent. None of the thinning agents cited currently in the patent or open literature meet all of the criteria as outlined above.
It is the primary object of this invention to develop an inexpensive thinning agent for the oxidation and cleavage reaction of ricinic compounds, which will allow for the successful thinning of the reaction mixture so as to allow for effective mixing and diffusion of by-products thus leading to higher yields and improved purity of the resulting sebacic acid product.
Another object of this invention is to develop a thinning agent that is easy to recover. Another object. of the invention is to develop a thinning agent that is non-volatile and meltable under the actual reaction conditions. Another object of the invention is to identify a thinning agent that is also prevents foaming. Another object of the invention is to develop a thinning agent that is stable to the reaction conditions employed in the oxidation.
This invention is an improved process for producing an organic carboxylic acid such as sebacic acid and an aliphatic alcohol such as octanol-2 from a ricinic compound such as castor oil and its derivatives. The process involves combining together in a reaction mixture an alkali in aqueous solution, a ricinic compound and a thinning agent and raising the temperature of the mixture to a level that will initiate a pyrolysis reaction to form a cleaved product from the ricinic compound. The thinning agent is present in sufficient amount to reduce solidification and foaming, while increasing the overall yield of the reaction and allowing for hydrogen to be liberated from the reaction. The thinning agent is oxidatively and/or thermally stable. The thinning agent can be selected from a group of low melting aliphatic organic isocarboxylic acids containing 5 to 13 carbon units (C5 to C13), such as isononanoic acids, 2 ethyl hexanoic acid, isovaleric acids, isodecanoic acids, and isotridecanoic acids. Also included are aliphatic organic isoaldehydes and isoalcohols from C 5 to C13 units, since these readily oxidize to the corresponding acid.
In one embodiment the reaction mixture is agitated to allow for uniform dispersion of the reactants. In one preferred embodiment the temperature of the reaction is about 200 to 350xc2x0 C. In another preferred embodiment the alkali is chosen from sodium hydroxide and potassium hydroxide. In another preferred embodiment the alkali has a pH greater than 9. Also cited in this disclosure is the reaction mixture for producing a cleaved product from a ricinic compound containing a thinning agent. In another embodiment, the process further includes adding an acid to the cleaved product in an amount sufficient to convert the cleaved product into a free acid.
In a preferred embodiment, the ricinic compound is castor oil, ricinoleate, a ricinic acid ester, a ricinoleic acid, a ricinoleic acid amide, a ricinoleic acid ester, a sulfonated ricinoleate, a ricinic ester, a ricinic alcohol, ricinoleyl alcohol, ricinoleyl alcohol ester, an alkali ricinoleate or a mixture thereof In a more preferred embodiment, the ricinic compound is glycerol tri-ricinoleate, a glyceryl tri (12-acetyl ricinoleate), glyceryl tri(12-hydroxystearate), glyceryl tri(12-acetoxystearate), methyl ricinoleate, ethyl ricinoleate, isoalcohol esters of ricinoleic acid e.g. 2-ethylhexanol ricinoleate, capryl ricinoleate, propylene glycol ricinoleate, or a mixture thereof
Isocarboxylic acids have been found to reduce the solidification and foaming tendency of the reaction mixture upon high temperature caustic oxidative cleavage of ricinic compounds such as castor oil and its derivatives leading to improved yield and purity of the product. This allows for the preparation of cleaved products such as di-sodium sebacate and octanol-2.
A big advantage of the isocarboxylic acids is that they are non-volatile in salt form, but are easily recovered after acid treatment of the products. They are resistant to thermal decomposition during the course of the reaction. The have the dual purpose of thinning the reaction mixture and reducing foaming. The alkali salts of isocarboxylic acids from C5 to C13 are the preferred thinning agents. Corresponding isoalcohols and isoaldehydes of these acids can be used also since they will readily oxidize to salts of carboxylic acids under the conditions employed in this caustic high temperature oxidative reaction. Some foaming may be produced as the result of the extra oxidative step. The process can be run in either the batch or continuous mode.
It has been found that it is not necessary to use catalysts such as alkali metal nitrates as suggested in the prior art (see Reis, U.S. Pat. No. 5,952,517) to produce high yields of sebacic acid and octanol-2 in this instant invention. Specifically, reaction solutions are added to a reactor which contains a heater and agitator. The reactor has a vapor pipe connected to a condenser for the collection of volatile products such as octanol-2. Subsurface steam line is also supplied. The aqueous alkali solution is preferably pre-heated to a temperature of 200-210xc2x0 C. To this reaction mixture an isocarboxylic acid (or the alcohol or aldehyde equivalent) and a ricinic compound are slowly added, preferably over a 1-4 hour period. A small amount of sub-surface steam is added during the first few hours of reaction. Volatile products, such as octanol are distilled from the reactor at a suitable temperature with the addition of steam. The temperature is raised to 200-350xc2x0 C. and maintained until hydrogen evolution has been completed. The distillate is recovered from the condensate receiver. The sodium salts from the reaction are diluted with water and acidulated with an acid such as sulfuric acid. The isocarboxylic acid thinning agent are recovered by vacuum distillation for reuse. Sebacic acid can be preferably extracted by hot water and crystallized to obtain the pure product.